Re: [PATCH 5/6 v4] cfq-iosched: CFQ group hierarchical scheduling and use_hierarchy interface

[Vivek Goyal <vgoyal@redhat.com>]

On Sat, Feb 12, 2011 at 10:21:47AM +0800, Gui Jianfeng wrote:
[..]
> >> +static struct cfq_group *
> >> +cfq_find_alloc_cfqg(struct cfq_data *cfqd, struct cgroup *cgroup, int create)
> >> +{
> >> +	struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
> >> +	struct cfq_group *cfqg = NULL;
> >> +	void *key = cfqd;
> >> +	struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
> >> +	unsigned int major, minor;
> >> +
> >> +	cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
> >> +	if (cfqg && !cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
> >> +		sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
> >> +		cfqg->blkg.dev = MKDEV(major, minor);
> >> +		goto done;
> >> +	}
> > 
> > Should we make this updation of this info hierarhical?
> 
> IMHO, it's fine to defer the updation when we really get the cfqg.

But if cfqg is alrady present, we will never hit the allocation path 
again. So if somebody creates 2-3 level deep hierarchy and does IO
only in the children cgroup, parent cgroups will potentially not get
device info updated hence no visible stats?

Thanks
Vivek